The use of a liquid level sensing probe is a known technique for monitoring the liquid level inside a receptacle, for example, a boiler of a pressurised steam generator. The sensing principle can be resistive or capacitive. Regarding using a capacitive sensing for detecting a liquid level within a receptacle, capacitive sensing probe can be located within a receptacle acting as a first electrode of a capacitive sensor, with a grounded conductive portion of the receptacle acting as a second electrode of the capacitive sensor. In such an arrangement, the liquid within the receptacle acts as a dielectric between the first plate and the second plate. A change in capacitance detected by a capacitive sensor can be utilised for determining a change in the liquid level contained within a receptacle. As the level of a liquid rises and falls in the receptacle, the dielectric effect of the liquid changes the effective capacitance of the capacitive sensor, which is detected by electronic circuitry coupled to the capacitive sensor.
Various methods have been proposed utilising capacitive sensing to detect the level of a liquid within a grounded receptacle (i.e. the receptacle in connected to earth). In conventional arrangements, isolated switched mode power supply (as depicted by FIG. 1) is required to fulfil the safety requirements of performing capacitive sensing. In such systems, there is an isolated barrier (e.g. a high frequency transformer) that can withstand from a few hundred volts to several thousand volts. However, these known techniques of using isolated switched mode power supplies are relatively expensive and complicated to implement.